Elections to Castlereagh Borough Council were held on 18 May 1977 on the same day as the other Northern Irish local government elections.

The election used three district electoral areas to elect a total of 19 councillors.

Note: "Votes" are the first preference votes.

1973: 3 x UUP, 2 x Alliance, 1 x United Loyalist 1977: 2 x Alliance, 2 x UUP, 1 x DUP, 1 x Independent Unionist 1973-1977 Change: DUP and Independent Unionist gain from UUP and United Loyalist 1973: 4 x UUP, 2 x Alliance, 1 x United Loyalist, 1 x Loyalist Coalition 1977: 3 x Alliance, 2 x UUP, 2 x DUP, 1 x Independent Unionist 1973-1977 Change: DUP (two seats) and Alliance gain from UUP, United Loyalist and Loyalist Coalition 1973: 3 x UUP, 1 x Alliance, 1 x Independent 1977: 2 x Alliance, 2 x UUP, 1 x DUP 1973-1977 Change: Alliance and DUP gain from UUP and Independent